I've been setting up rooms for many years and this is the most challenging I've encountered. I could use some advice. After hours & iterations of room arrangement & prep I have it within +/- 2 DB of flat from 150 Hz to up. So far so good. The problem is two strong room modes: +9 dB @ 100 Hz, and -9 dB @ 50 Hz. If I can kill these modes the room will sound better than most recording studios. It already sounds spectacular with music that doesn't have deep bass - voice, piano, chamber music, etc. But there's no deep bass. It's -3 dB @ 60 Hz and drops from there.

What is the likelihood that tube traps can give -9 @ 100 Hz and kill my 100Hz mode? Any other options?

Is there any room treatment that can restore the deep bass? If a subwoofer is the only solution, what would you recommend?

The speakers are Magnepan 3.6/R. In my last room, which was bigger, I was able to arrange & prep to get flat response (-2 dB) down to 32 Hz and I didn't need a sub. This room measures 15' deep by 16.7' wide by 7' high. I know it's too small for good bass, but it's all I've got. I've killed reflections and gotten awesome clean imaging with thick RPG Profoam, but it has virtually no effect on room modes.

I'm measuring response by playing 1/3 octave warble tones with an SPL meter at the listener position. To validate the meter I've done the same test with good studio mics (Rode NT-1As) and corrected for mic response, same results as the SPL meter.